Update publish-to-pypi.yml

A follow on to #784. This change also updates the
action to publish to PyPI (official) to use a released
version of gh-action-pypi-publish instead of master.
This change also makes the secret variable stand out
a little more with all caps.

.github/workflows/publish-to-pypi.yml

@@ -24,6 +24,6 @@ jobs:
 
     - name: Publish distribution to PyPI
       if: startsWith(github.ref, 'refs/tags')
-      uses: pypa/gh-action-pypi-publish@master
+      uses: pypa/gh-action-pypi-publish@release/v1
       with:
-        password: ${{ secrets.pypi_password }}
+        password: ${{ secrets.PYPI_PASSWORD }}